1. Fields of the invention
The present invention relates to a method for manufacturing light emitting devices.
2. Descriptions of Related Art
The first light emitting diode (LED) product was developed in 1968. Then LED of various colors have been invented in optoelectronic industries. In 1993, Japan Nichia Corporation demonstrated blue and green LEDs with higher luminous efficiency and this caused development of full-color LED products. The existence of GaInN based blue and green LEDs quickly led to the development of the first white light LED. Since mass production of white light LED by Nichia corporation in 1996, global LED manufacturers speed up the pace of mass production of white light LED. White light LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems so that they nearly completely replace traditional incandescent lighting available now in virtually all applications. White light LEDs have become energy saving and environmental friendly lighting. Moreover, LED has features of high shock resistance, long life and less power consumption etc. Thus more and more outdoor lighting devices or displays such as traffic signs or outdoor electronic billboards use LED as light emitting elements. Furthermore, under the trends of energy saving and carbon reduction, LED has become mainstream of backlight of liquid crystal displays because LED meets the requirements of high brightness and low power consumption. LED not only comply with the requirements of environmental protection but also provides high-brightness illumination, especially white light LED which has more applications.
Traditional white light sources made from sold semiconductor are divided into three groups. The first white-light module composed of mixed red, blue and green LED chips has advantages of high luminous efficiency and high color rendering index. The LED chips with different colors are made from different epitaxial materials so that the voltage of the light module varies according to electrical properties of the chips. Thus the cost of the white light module is high, the design of the control circuit is quite complicated, and poor color mixing. The second group uses YAG phosphor powder to convert light from a blue LED to white light. This is the mainstream of white light LED manufacturing on the market. The technology features on coating a layer of optical adhesive mixed with YAG phosphor powder on a blue LED chip so that light from the blue LED emits into the optical adhesive and excites the YAG phosphor powder so as to obtain a light having a wavelength ranging from 400 to 530 nm. Part of blue light emitted from the blue LED chip is mixed with light excited from the phosphor powder to produce light that appears white. The third group LED is fabricated using both RGB (red, green and blue) phosphors and UV LEDs. The UV LEDs emit blue-violet color and phosphors in an optical adhesive are excited to produce white light with three wavelengths.
As to the packaging of white light LED, the heat dissipation problem has great influence on the lifetime while developing high power and large area LED lighting module. The LED is packaged by glue dispensing, encapsulation, molding, etc. Epoxy is the most common used material for packaging for economic concern. However, if the epoxy becomes sticky, the phosphor layer will have unavoidable defects such as bubbles, notches, dark spots and phosphor precipitation. Moreover, the phosphor layer is not a smooth and flat surface so that the colored light can't maintain the uniformity. Thus the white light LED products have color difference at different angles. Along with the improvement of phosphors or encapsulant materials, the luminous efficiency and light conversion efficiency of white light LED have been improved. Yet the white light
LED still got many problems. For example, the thickness of the phosphor layer keeps increasing. Or external electrodes of the white light LED have been covered by phosphors or encapsulant materials and this leads to bad electrical contacts. Furthermore, the phosphor layer still have non-flat surface and the color uniformity issue is still a problem.
Thus there is a need to invent a method for manufacturing light emitting devices that not only improves electrode coverage but also reduce the thickness of the phosphor layers. Thus the thickness of lighting devices with LED is effectively reduced.